To save energy, there is an effort to replace incandescent light bulbs with more efficient alternative light source. One alternative is the fluorescent lamp, especially the compact fluorescent lamp. While fluorescents addresses the energy issue, it introduces another concern. Fluorescent lamps, whether intended as a screw-in replacement for incandescent or as the traditional tube style, contain toxic mercury. This runs counter to a parallel effort to reduce toxic elements from our waste stream.
Light emitting diodes (LED) are an alternative that do not contain mercury, are far more efficient than incandescent bulbs, and can surpass the efficiency of fluorescent bulbs. To be compatible with normal AC line voltage, however, special circuitry is needed to convert the AC line voltage into a DC current suitable for driving the LEDs.
Referring to FIG. 1, there is shown a simple driver circuit of the prior art consisting of a rectifier (D), an energy storage capacitor (C), and a linear current regulator (REG) that is used to convert the AC supply voltage (VAC) to a constant current DC source for driving LEDs. A drawback of this circuit is the power dissipated in the current regulator (REG), which varies with line and load voltage, resulting in inefficient operation. A further disadvantage is poor power factor as high current pulses are drawn from the AC source during peak charging of the energy storage capacitor (C). These pulses generate line current harmonics which may not be in compliance with regional regulations. Additionally, the storage capacitor (C) is a further drawback of this circuit since it typically is a high voltage electrolytic type which is bulky and subject to failures in high temperature environments.
Referring to FIG. 2, there is shown a switching regulator circuit of the prior art employing inductors to convert AC voltage (VAC) into DC current. Although the circuit with inductors is more efficient, a downside includes complex design, relatively high printed circuit board area, conducted EMI, and expensive components. Components include inductors (L1 and L2), high voltage electrolytic capacitors (CS), high voltage power transistors (Q), high voltage film capacitors (CHV), and high voltage-fast recovery rectifiers (D2). Electrolytic capacitors (CS) are especially troublesome as they are bulky and can be a major contributor to system failure. To provide power factor correction (PFC) additional circuitry is needed. An input filter is needed to control injected noise back into the AC power line. To provide compatibility with lamp dimmers, yet more circuitry is required.
Referring to FIG. 6, there is shown an LED driver circuit of the prior art using a central sequencer, with the sequencer depicted as a functional block. There are several methods to implement the sequencer, the straightforward approach is to monitor the rectified AC (VRAC). When VRAC passes predetermined voltage levels the appropriate current control element is enabled or disabled. The drawback of this method is that the predetermined voltage levels might not match the forward voltage drop of the LED segments. This is especially true over temperature as LEDs exhibit an approximate −2 mV/° C. temperature variation. Multiplied by the number of serially connected LEDs in the string can result in significant mismatch. Premature enabling of a downstream control element with the simultaneous disabling of its' upstream neighbor occurs when there is not enough voltage to forward bias the LED segment, resulting in gaps in the input current waveform which may produce conducted EMI problems. In addition, it results in underutilization of the LEDs. Conversely, if the mismatch enables the control elements too late, it results in overlap and current spikes. Other drawbacks include the need for additional circuitry, another high voltage connection to VRAC, and the need to employ LEDs with consistent forward voltage drop.
An ideal LED driver circuit would consist of a small, low cost integrated circuit, driving a string of low cost, low to medium brightness LEDs (instead of high cost, high brightness LEDs) and requiring no inductors, no capacitors (especially electrolytic), no heat sink, and a few inexpensive components such as a bridge rectifier and a few resistors to configure and optimize the performance of the driver, while providing high efficiency, high power factor, lamp dimming compatibility, good line and load regulation, low line current harmonics, and low conducted EMI. Accordingly the present invention disclosed herein describes such a driver circuit.